On August 15, 1977, at 11:16pm (Eastern Daylight Savings Time), the “Big Ear” radio telescope at Ohio State University detected a narrowband signal that was could be a transmission of some form. The codes used in their printout reflected the intensity by using numbers and letters. Sprinkled among their 1s, spaces and occasional 6s was the 6-letter string 6EQUJ5, representing 72 seconds that just might be a transmission from an alien intelligence.

The article I linked above is very technical (I must confess to not understanding quite a bit of it), but it’s still fascinating. If you don’t want to read the technical and scientific information, the key questions are:

Was it a message of some kind?
The short answer is – we don’t know, and there’s no way to tell. The way in which it was recorded detects only its presence and how long it lasted for; it doesn’t tell us whether there was any meaningful signal there.
Basically, the computer was looking for the presence of a signal and not its content – imagine it like the signal strength bars on a mobile phone. The signal bars tell you how strong your connection is – not who’s calling, or whether your phone has any credit.

Why didn’t we see it before?
The signal could have been broadcast for hundreds – or perhaps thousands – of years before being detected. However the previous detection was wideband because they were looking for different types of cosmic events.

I’m confused by this “wideband” and “narrowband” stuff – why wouldn’t the signal be detected in “wideband”?
Basically, wideband means you’re scanning a larger part of the radio spectrum. This is what you’d use to scan for “cosmic events” – supernovae, pulsars and so on. Because it scans so wide an area, even a strong signal (such as a TV or radio station) would be a drop in the bucket compared to the surrounding interference – like trying to receive every radio station being broadcast in the world at once.
The switch to narrowband – which they undertook in 1973 – was done as a means of keeping the telescope in use for something, but in a less human-resources intensive way, as due to budget cutbacks whatever they used the telescope for was essentially going to be “hobbyist”: by which I mean that nobody was getting paid for it!

How do we know it’s not a hoax?
You’d need to be a competent radio engineer, own a large battery and then have no idea whether the signal would actually look right at the other end. Dr Jerry Ehman, who discovered the signal, doesn’t address this, but I believe that it would be so hard to do the probability of the signal being a hoax is so small as to be below consideration.

So where did it come from?Roughly speaking, the signal was detected coming from the direction of the constellation Sagittarius. Professional astronomers can find more information on the article above.

Will we ever see it again?
The same part of the sky has been scanned since, at the same frequency, and it hasn’t been detected again, but that’s not to say that it won’t be in the future.